A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., e.g., including part of, one, or several dies) on a substrate (e.g., e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ                          NA              PS                                                          (        1        )            where λ is the wavelength of the radiation used, NAPS is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NAPS or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation sources are configured to output a radiation wavelength of about 13 nm. Thus, EUV radiation sources may constitute a significant step toward achieving small features printing. Such radiation is termed extreme ultraviolet or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. Along with useful EUV in-band radiation, EUV radiation sources may produce almost equal (and sometimes more) undesirable out-of-band infrared (“IR”) and deep ultraviolet (“DUV”) radiation.
In a lithographic apparatus, the substrate is held very rigidly on the substrate table so that its position can be accurately known even when the substrate table undergoes high accelerations during its scanning motion. In existing machines, the substrate holder, or chuck, comprises a pimpled surface surrounded by a wall. The substrate rests on the wall and pimples and the space behind it is evacuated so that air pressure above provides a strong clamping force holding the substrate in place. Further details of such a substrate holder can be found in EP-A-0,947,884, incorporated herein by reference.
The above type of substrate holder has proven effective for present day lithographic apparatus. However, as described above to meet the ever-present demand for imaging features of reduced size, it is necessary to reduce the wavelength of the radiation used for the projection beam. Thus, whilst current devices use ultraviolet radiation, e.g., with a wavelength of 248 nm, 193 nm or 157 nm, improved resolution requires the development of lithographic apparatus making use of extreme ultraviolet (EUV) radiation (i.e., with a wavelength of less than about 50 nm), x-rays, electrons or ions. These proposed types of radiation all share the requirement that the beam path, or at least substantial parts of it, must be kept in vacuum. Thus, without any air pressure above the substrate, the conventional vacuum-based substrate holder cannot function.
Similar requirements also need to be met in mask writing, mask cleaning and mask inspection apparatus and so chucks suffer from the same problems as the lithographic projection apparatus.
It has therefore been proposed to use electrostatic forces to hold the substrate onto the substrate table using an electrostatic chuck. To effect this, a potential difference is applied across a dielectric material with electrodes. In one example of such an electrostatic chuck (or clamp) a potential difference is applied between an electrode on the substrate and an electrode in or on the substrate table. When the potential difference is applied, the electrode of the substrate and the electrode of the table become oppositely charged and attract each other with sufficient force to clamp the substrate in place.
U.S. 2002/0044267 discloses a holder that comprises a platen made of the glass ULE™ on which a holder is positioned. The holder may be an electrostatic chuck as disclosed, for example, in U.S. Pat. No. 5,221,403, U.S. Pat. No. 5,835,333 or U.S. Pat. No. 5,835,334.
EP-A1-1,359,469 discloses using a dielectric with certain properties and suggests use of glass or glass ceramics.
WO 2011/001978 and EP-A1-1, 909, 308 disclose an electrostatic clamp in accordance with the pre-characterising section of claim 1.